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          <th width="60%" align="center">Chapter 10. Secondary Databases</th>
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    <div class="sect1" lang="en" xml:lang="en">
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          <div>
            <h2 class="title" style="clear: both"><a id="joins"></a>Database Joins</h2>
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      <p>
        If you have two or more secondary databases associated with a primary
        database, then you can retrieve primary records based on the intersection of 
        multiple secondary entries. You do this using a
        <span><tt class="classname">JoinCursor</tt>.</span>
        
    </p>
      <p>
        Throughout this document we have presented a 
            <span>class</span>
            
        that stores 
            <span>inventory</span>
        information on grocery
            
            
        That 
            <span>class</span>
            
        is fairly simple with a limited
        number of data members, few of which would be interesting from a query
        perspective. But suppose, instead, that we were storing 
        information on something with many more characteristics that can be queried, such
        as an automobile. In that case, you may be storing information such as
        color, number of doors, fuel mileage, automobile type, number of
        passengers, make, model, and year, to name just a few.
    </p>
      <p>
        In this case, you would still likely be using some unique value to key your
        primary entries (in the United States, the automobile's VIN would be
        ideal for this purpose). You would then create a 
            <span>class</span>
            
        that identifies
        all the characteristics of the automobiles in your inventory. 
        
        <span>
            You would
            also have to create some mechanism by which you would move instances of
            this class in and out of Java <tt class="literal">byte</tt> arrays. We
            described the concepts and mechanisms by which you can perform these
            activities in <a href="DBEntry.html">Database Records</a>.
        </span>
    </p>
      <p>
        To query this data, you might then create multiple secondary databases,
        one for each of the characteristics that you want to query. For
        example, you might create a secondary for color, another for number of
        doors, another for number of passengers, and so forth. Of course, you
        will need a unique 
            <span>key creator</span>
            
        for each such secondary database. You do
        all of this using the concepts and techniques described throughout this
        chapter.
    </p>
      <p>
        Once you have created this primary database and all interesting
        secondaries, what you have is the ability to retrieve automobile records
        based on a single characteristic. You can, for example, find all the
        automobiles that are red. Or you can find all the automobiles that have
        four doors. Or all the automobiles that are minivans. 
    </p>
      <p>
        The next most natural step, then, is to form compound queries, or joins.
        For example, you might want to find all the automobiles that are red,
        and that were built by Toyota, and that are minivans. You can do this
        using a 
            <span><tt class="classname">JoinCursor</tt> class instance.</span>
            
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="joinUsage"></a>Using Join Cursors</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
            To use a join cursor:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    Open two or more 
                        <span>secondary cursors. These </span>
                    cursors 
                        
                        <span>for</span> 
                    secondary databases that are associated with
                    the same primary database.
                </p>
            </li>
            <li>
              <p>
                    Position each such cursor to the secondary key
                    value in which you are interested. For example, to build on
                    the previous description, the cursor for the color
                    database is positioned to the <tt class="literal">red</tt> records
                    while the cursor for the model database is positioned to the
                    <tt class="literal">minivan</tt> records, and the cursor for the
                    make database is positioned to <tt class="literal">Toyota</tt>.
                </p>
            </li>
            <li>
              <p>

                    Create an array of <span>secondary</span> cursors, and
                    place in it each of the cursors that are participating in your join query. 
                    

                </p>
            </li>
            <li>
              <p>

                    Obtain a join cursor. You do this using the 
                        <tt class="methodname">Database.join()</tt> 
                         
                         
                    method.  You must pass this method the array of secondary cursors that you
                    opened and positioned in the previous steps.
                </p>
            </li>
            <li>
              <p>
                    Iterate over the set of matching records
                        <span>using <tt class="methodname">JoinCursor.getNext()</tt></span>
                    until
                    <span><tt class="classname">OperationStatus</tt> is not <tt class="literal">SUCCESS</tt>.</span>
                    
                </p>
            </li>
            <li>
              <p>
                    Close your <span>join</span> cursor.
                </p>
            </li>
            <li>
              <p>
                    If you are done with them, close all your <span>secondary</span> cursors.
                </p>
            </li>
          </ul>
        </div>
        <p>
            For example:
        </p>
        <a id="java_index9"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.JoinCursor;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
import com.sleepycat.db.SecondaryCursor;
import com.sleepycat.db.SecondaryDatabase;

...

// Database and secondary database opens omitted for brevity.
// Assume a primary database handle:
//   automotiveDB
// Assume 3 secondary database handles:
//   automotiveColorDB  -- index based on automobile color
//   automotiveTypeDB  -- index based on automobile type
//   automotiveMakeDB   -- index based on the manufacturer
Database automotiveDB = null;
SecondaryDatabase automotiveColorDB = null;
SecondaryDatabase automotiveTypeDB = null;
SecondaryDatabase automotiveMakeDB = null;

// Query strings:
String theColor = "red";
String theType = "minivan";
String theMake = "Toyota";

// Secondary cursors used for the query:
SecondaryCursor colorSecCursor = null;
SecondaryCursor typeSecCursor = null;
SecondaryCursor makeSecCursor = null;

// The join cursor
JoinCursor joinCursor = null;

// These are needed for our queries
DatabaseEntry foundKey = new DatabaseEntry();
DatabaseEntry foundData = new DatabaseEntry();

// All cursor operations are enclosed in a try block to ensure that they
// get closed in the event of an exception.

try {
    // Database entries used for the query:
    DatabaseEntry color = new DatabaseEntry(theColor.getBytes("UTF-8"));
    DatabaseEntry type = new DatabaseEntry(theType.getBytes("UTF-8"));
    DatabaseEntry make = new DatabaseEntry(theMake.getBytes("UTF-8"));

    colorSecCursor = automotiveColorDB.openSecondaryCursor(null, null); 
    typeSecCursor = automotiveTypeDB.openSecondaryCursor(null, null); 
    makeSecCursor = automotiveMakeDB.openSecondaryCursor(null, null); 

    // Position all our secondary cursors to our query values.
    OperationStatus colorRet = 
        colorSecCursor.getSearchKey(color, foundData, LockMode.DEFAULT);
    OperationStatus typeRet = 
        typeSecCursor.getSearchKey(type, foundData, LockMode.DEFAULT);
    OperationStatus makeRet = 
        makeSecCursor.getSearchKey(make, foundData, LockMode.DEFAULT);

    // If all our searches returned successfully, we can proceed
    if (colorRet == OperationStatus.SUCCESS &amp;&amp;
        typeRet == OperationStatus.SUCCESS &amp;&amp;
        makeRet == OperationStatus.SUCCESS) {

        // Get a secondary cursor array and populate it with our
        // positioned cursors
        SecondaryCursor[] cursorArray = {colorSecCursor,
                                         typeSecCursor, 
                                         makeSecCursor};

        // Create the join cursor
        joinCursor = automotiveDB.join(cursorArray, null);

        // Now iterate over the results, handling each in turn
        while (joinCursor.getNext(foundKey, foundData, LockMode.DEFAULT) ==
                        OperationStatus.SUCCESS) {

            // Do something with the key and data retrieved in
            // foundKey and foundData
        }
    }
} catch (DatabaseException dbe) {
    // Error reporting goes here
} catch (Exception e) {
    // Error reporting goes here
} finally {
    try {
        // Make sure to close out all our cursors
        if (colorSecCursor != null) {
            colorSecCursor.close();
        }
        if (typeSecCursor != null) {
            typeSecCursor.close();
        }
        if (makeSecCursor != null) {
            makeSecCursor.close();
        }
        if (joinCursor != null) {
            joinCursor.close();
        }
    } catch (DatabaseException dbe) {
        // Error reporting goes here
    }
} </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="joinconfig"></a>JoinCursor Properties</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
            You can set <tt class="classname">JoinCursor</tt> properties using the
            <tt class="classname">JoinConfig</tt> class. Currently there is just one property that you can
            set:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    <tt class="methodname">JoinConfig.setNoSort()</tt>
                </p>
              <p>
                    Specifies whether automatic sorting of input cursors is disabled. The cursors are sorted from the
					one that refers to the least number of data items to the one that refers to the most.
                </p>
              <p>
					If the data is structured so that cursors with many data items also share many common elements,
					higher performance will result from listing those cursors before cursors with fewer data
					items. Turning off sorting permits applications to specify cursors in the proper order given this
					scenario.
				</p>
              <p>
					The default value is <tt class="literal">false</tt> (automatic cursor sorting is performed).
				</p>
              <p>
                    For example:
                </p>
              <a id="je_index10"></a>
              <pre class="programlisting">// All database and environments omitted
JoinConfig config = new JoinConfig();
config.setNoSort(true);
JoinCursor joinCursor = myDb.join(cursorArray, config); </pre>
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